Such a method is already known from EP-A-84902732.1, where the U.S. Pat. No. 4,559,828 reference class comprises frequency spectra and where pattern recognition and detection means are adapted for calculating the probability, for each new frequency spectrum, that the latter is associated with a class other than the reference class, whereby an abnormal operational state of the machine is detected when this probability exceeds a predetermined limit.
The known method is advantageous in that no interpretation of frequency spectra needs to be made as long as each sensed frequency spectrum belongs to the reference class. Only when abnormal operational states occur does the frequency spectrum need to be studied more closely. For simple machines, each peak in the spectrum can be attributed to a given function or to a given machine part, and even very small functional changes can thus be discovered at an early stage.
In more complicated machines with at least two rotating machine parts operating at different revolutionary rates and in mutual co-action, particularly via different mechanisms, as is the case in jet engines, the known method cannot be used without complications. Accordingly, each spectrum peak must be analyzed with relation to its origin. This is complicated, and in addition is not always possible, since different machine parts in certain combinations of revolutionary rates can give rise to coinciding spectrum peaks.